The present invention broadly relates to video trackers for tactical system applications, and deals more particularly with a tracker having both correlation and centroid video processors.
Tactical system applications for video trackers require high performance even where background and foreground clutter objects compete with the target of interest. Additionally, these systems must satisfactorily perform under dynamic conditions where the relative aspect angles and range to the target are continuously changing, and where image roll about the track sensor line-of-sight may be quite severe.
Video tracking processors have been devised in the past which utilize a variety of processing techniques or algorithms such as centroid, area balance, edge and numerous correlation tracking implementation concepts. Most of these video tracking processors posseses characteristics which render them unsuitable for use in combination with each other in a "dual mode" operation. For example, edge and area balance algorithms are either noisy or lack well defined error scale factors independent of target shapes. Many correlation algorithm implementation do not provide satisfactory performance when tracking the scene changes on a dynamic basis; consequently, when these types of correlation algorithms are used in conjunction with other tracking processors in a dual mode role, there is excessive dependence on the alternate tracking algorithm. Additionally, when targets which are moving relative to the background are being tracked, it is extremely desirable to position a tracking gate around the target of interest thereby excluding the stationary background. However, techniques for deriving meaningful track gates for many correlation processing algorithms do not exist or rely on externally derived data that are measures of parameters which are not fundamental or relevant to correlation processing.
Video tracking processors of the type described above are normally designed to accept a specific video scan and interlace video format. Thus, if tracking sensors are employed which yield a video format different from that normally accepted by the tracking processor, the processor design must be modified or, alternatively, the input video format must be scan converted to the required video format in order to be processed. In event that scan conversion is required, a significant processing delay is introduced, thereby limiting the track loop bandwith and degrading the tracking performance potentioal under dynamic conditions. Additionally, the scan conversion process often obscures scene spatial sampling intrinsic to the senor being used and introduces video artifacts and/or spatial and temporal sampling interactions that are deleterious to the generation of proper tracking error measurements.
Ideally, it would be desirable to provide a video tracker having two video tracking processors of different types each of which is particularly suited to provide high performance in a given set of tactical applications, and which is readily suitable for use with various scanning and interlace formats.
Both centroid and correlation type video tracking processors are well known in the art. For example, U.S. Pat. No. 4,133,044 issued Jan. 2, 1979 to Fitts discloses a video correlation tracker which employs a recursive reference to calculate tracking error computations. The correlation tracker disclosed in the Fitts patent includes a circuit for generating a reference map in pixel format. Reference map pixel information derived from previous video frames is stored in a recursive memory to allow the calculation of azimuth and elevation optimal weighting values for each pixel in the field of view. The difference between the intensity value for each video pixel being received during the current frame and the intensity value of the corresponding reference map pixel is multiplied by an appropriate weighting function. Each resultant product is then combined in an accumulator to form azimuth and elevation correlation error signals which are composite indications of the frame-to-reference correlation over the designated track gate area which can be as large as the entire field of view (FOV), less a one pixel border. The weighting factors for each pixel are also combined and accumulatively added over the entire image plane to form three adaptive scale factors at the end of each frame which are combined with the azimuth and elevation correction error signals to eliminate cross coupling and generate cross-coupling-free correlation error signals.
The video correlation tracker disclosed in the Fitts patent mentioned above is particularly well suited to dynamic applications where scene aspect angles, range and scene roll orientation about the line-of-sight change rapidly.